1. Technical Field
The present invention generally relates to searching data relating to a spatial region using a prioritized search order, and more particularly to a Terrain Awareness Warning System (xe2x80x9cTAWSxe2x80x9d) for use by an aircraft for searching terrain elevation data for a geographic area to determine aircraft terrain clearance.
2. Background Art
Various systems are known in the art which provide warnings and advisory indications of hazardous flight conditions. Among such systems are systems generally known as Ground Proximity Warning Systems (xe2x80x9cGPWSxe2x80x9d) which monitor the flight conditions of an aircraft and provide a warning if flight conditions are such that inadvertent contact with terrain is imminent. Among flight conditions normally monitored by such systems are radio altitude and rate, barometric altitude and rate, air speed, and flap and gear positions. These parameters are monitored and an advisory signal and/or warning signal is generated when the relationship between the parameters is such that terrain impact is likely to occur.
Other systems improve upon earlier GPWS by utilizing ground-based navigational information. By monitoring stored terrain data and providing modified ground proximity warnings, more accurate warnings may be provided. U.S. Pat. Nos. 4,646,244, and 4,675,823 each disclose terrain advisory systems which utilize ground-based navigational systems and stored terrain data to provide various ground proximity warnings in relation to the position of the aircraft. U.S. Pat. No. 5,839,080 discloses a Global Positioning System (xe2x80x9cGPSxe2x80x9d) and stored terrain data to provide warning indications.
Satellite-based navigational systems, such as GPS, which can track longitude, latitude, altitude, groundtrack, and ground speed, are becoming an important and reliable source of information for aircraft. An aircraft""s Forward Looking Terrain Avoidance (xe2x80x9cFLTAxe2x80x9d) system looks ahead of the aircraft during flight along and below the aircraft""s lateral and vertical flight path to provide suitable alerts if a potential threat exists of the aircraft colliding or coming too close to terrain. The computation involves searching through a terrain database for terrain cells that are within the search area and violate the Required Terrain Clearance (xe2x80x9cRTCxe2x80x9d). The RTC is the value set by the Federal Aviation Administration as the permitted flight xe2x80x9cfloorxe2x80x9d for various phases of aircraft flight. The RTC indicates the clearance distance from terrain below which the aircraft should not fly. Searching the search area and finding the cells in violation is expensive in both processor and memory resources.
Two common methods of searching terrain data are sequential and radial. Both of these methods suffer from the deficiency that they expend precious processor and memory resources. For the sequential search method, some common deficiencies include: First, every cell in a rectangular area encompassing the search area is searched, even those cells which are outside the search area boundary. This requires a determination of whether a cell is within the search area or not, which could be complicated and expensive. Second, a large data buffer is needed to store the cells found during the search, the dimension of the buffer needed is difficult to determine accurately because the upper bound could equal the size of the search area which varies by the speed of the plane. Third, sorting the cells in the result buffer by distance to the aircraft position is expensive, especially if a large number of cells are returned in the search and if both distance and bearing are considered.
For the radial search method, some common deficiencies include: First, many possible flight directions are searched, even some of those which are highly unlikely. This requires expensive processing time to be spent on searching all flight directions even when the aircraft is flying straight. Second, because the search is performed in a plurality of radial arms originating at the same point and extending in a fan shape over the underlying data sectioned into a square grid, many cells near the origin of the arms are searched numerous times, and some cells farther from the origin are potentially missed if the radial arms become too far apart at their extents. Third, the position of each point on each radial arm is calculated and terrain data is searched throughout the full database. This is complicated and expensive. Fourth, a large data buffer is needed to store the cell identities found during the search, and the dimension of the buffer needed is difficult to determine accurately because the upper bound could equal the full number of the search points on all radial arms. Fifth, in systems which sort the results, sorting the cells in the result buffer by distance to the aircraft position is expensive, especially if a large number of cells is returned in the search and if both distance and bearing are considered.
Both conventional systems require extensive processor search time and memory storage for crucial Terrain Awareness and Warning systems (xe2x80x9cTAWSxe2x80x9d), for which the costs of developing, repairing and maintaining are increasing. The additional time required for these searches also reduces the time available for the processor to perform other tasks and lengthens the time in which search results can be made available to a pilot of the aircraft.
The present invention relates to methods, apparatus and a system for searching spatial data in a prioritized manner. In a general form of the invention, spatial data along and adjacent to a search vector is searched by storing a portion of a larger spatial information database into a smaller memory buffer and searching the buffer in a predetermined prioritized order. Different from conventional methods of searching spatial data, particular embodiments of the present invention search the data in the memory buffer in a non-linear order along and adjacent to a search vector, comparing spatial data values stored in select memory cells of the buffer with a predetermined search criteria. The identity of each cell searched which stores a value meeting the search criteria is included in an alert list which, when the length of the list equals a predetermined list length, is used to calculate an alert status for each cell on the list. Each cell in the memory buffer is searched no more than once for each alert cycle. Subsequent cycles may search using the same predetermined search order along the search vector, or a different predetermined search order.
Embodiments of the present invention are also particularly useful for searching geographic terrain data for use with TAWS for aircraft. By reference to a satellite navigation reference signal and possibly other signals which provide an indication of an aircraft""s altitude, location and direction of travel, a processor determines a search vector in relation to the aircraft""s coordinates. The processor accesses a geographic terrain information database and stores elevation data corresponding to the spatial area portion of the database representing the geography over which the aircraft is traveling in a memory buffer. Using a predetermined prioritized order for searching the cells of the memory buffer along the search vector, the processor compares the aircraft""s projected altitude at each cell with the elevation value stored in the cell plus an appropriate RTC value for the flight phase (elevation+RTC=xe2x80x9cclearance elevationxe2x80x9d). Particular embodiments of the invention employ a non-linear prioritized order. For all searched cells where the projected altitude of the aircraft is less than the clearance elevation at that cell, the identities of the cells are stored in an alert list until the list is full or the predetermined order is finished. When a predetermined number of alert list elements have been recorded, the processor, in association with a look-ahead warning generator, provides either caution or warning alert indicators to a pilot of the aircraft based upon a separate time-related analysis of the data. The alert indicators may be visual, aural, or both. Embodiments of the invention employ a select number of predetermined search patterns between which a selection is made and implemented for an alert cycle depending upon an exterior criteria such as a pilot""s actions or the aircraft""s movements. In particular embodiments of the invention, the predetermined search pattern searches a few cells in the direction of the flight path, then back-steps and searches unsearched cells adjacent the flight path, then returns to the flight path and searches at least one next unsearched cell in the flight path, then again searches unsearched cells adjacent the flight path.
Particular advantages of the present invention are found in significantly reduced processing and computation requirements for system resources employing embodiments of the present invention for use in spatial data searching. By storing a relevant portion of the larger terrain information database in a smaller memory buffer, less data needs to be accessed and dismissed to find relevant data, significantly reducing the calculations needed. By searching the cells of the memory buffer no more than once per alert cycle and in a predetermined prioritized order, the alert list is automatically prioritized and cell data is not searched repeatedly. By searching each memory buffer cell using a single simple comparison, searching the search region only until the alert list reaches a predetermined number of cells, and performing a time-related analysis on the significantly shorter list of cells rather than the entire database, even further processing and calculation savings is achieved as well as providing and displaying relevant alert information to the pilots more quickly.
The foregoing and other features and advantages of the present invention will be apparent from the following more detailed description of the particular embodiments of the invention, as illustrated in the accompanying drawings.